1. Field of the Invention
This invention relates generally to rolling mills and, more particularly, to a four-high rolling mill which can be converted between a hot mill configuration for rolling relatively narrow products, such as hot mill band or plate in coil form, and a plate mill configuration for rolling relatively wide products, such as wide plate.
2. Description of the Prior Art
Many different types of rolling mills are known in the art for rolling various products, such as steel plates, coils, strips and the like. Typical examples of rolling mills include two-high, three-high and four-high mills. As the names indicate, these classifications are based on the manner of arranging the rolls in the housings. A four-high mill has four rolls arranged one above the other. Four-high mills are used for rolling flat material, like sheets and plates, and represent a special type of mill for both hot and cold rolling. In a four-high mill, large backup rolls are used to reinforce smaller work rolls. Either the backup or work rolls may be driven. Four-high mills generally resist the tendency of long work rolls to deflect and permit the use of small diameter work rolls for producing wide plates and hot or cold rolled strip and sheets of uniform gauge.
A typical four-high mill consists of a mill stand formed by a pair of spaced apart housings mounted on a base. Each housing consists of a pair of upright standards or legs having an opening or window which extends therebetween. A pair of work rolls and a pair of backup rolls are rotatably carried in the mill stand. The window serves as a receptacle for the bearing blocks of the work and backup rolls. A hold-down or screw-down mechanism is used to adjust the distance between the work rolls. The screw-down mechanism consists of a single pair of screws with one screw on each side of the mill stand and extending down through the top of the mill stand. The screws push down on the opposed bearing blocks of the top backup roll and thus adjust the distance between the work rolls. On small mills where adjustments are only occasionally made, these screws are operated by hand with spanner bars. In larger mill stands, hydraulic or electric drives may be used to turn the screw-down mechanism. In typical rolling operations, a series of mill stands are spaced closely together in one continuous line so that the product is being rolled concurrently in more than one mill stand as it passes in a straight line from one stand to the next. Such an arrangement is known as a tandem mill.
Four-high mills for rolling heated slabs are typically used for two different kinds of rolling applications. These are known in the art as "hot mills" and "plate mills" and are typically thickness and width dependent. A hot mill has relatively short work and backup rolls and is used to roll products having a thickness capable of being coiled and a maximum width of about 80 inches, with a typical width of about 56 inches. A plate mill has relatively longer work and backup rolls and is used to roll a thicker product, such as 3/4 inch and thicker, and much wider products, for example, about 132 inches. Because of their width restriction, hot mills are not capable of being used as plate mills. Also, plate mills cannot effectively be used as hot mills because trying to roll a narrow product causes the relatively longer backup rolls of the hot mill to deflect or arc under the pressure of the screw-down mechanism thus causing the backup rolls to lose their stiff contact with the entire length of the work rolls. Therefore, it is standard practice in the art to have separate mill stands for hot mill and plate mill rolling operations. For mill owners, the requirement to have separate hot mills and plate mills can be a large drain on operating costs unless the product mix is sufficient to maintain both types of rolling operations simultaneously. Also, the large initial expenditure of capital needed to purchase both hot mills and plate mills is quite burdensome.
Therefore, it is an object of the present invention to provide a four-high mill stand which can be quickly and easily converted between hot mill and plate mill configurations.
It is a further object of the invention to provide a screw-down assembly which can be used with either the present invention or which can be economically retrofitted onto an existing mill stand to allow for conversion of the mill stand between hot mill and plate mill configurations.
It is also an object of the present invention to provide a method for converting a mill stand between hot mill and plate mill configurations.